Transfer of cytoplasmic male sterility by selection for streptomycin resistance after protoplast fusion in Nicotiana

Cytoplasmic male sterility (CMS) in Nicotiana is located on mitochondrial DNA (mtDNA) on which no selectable mutation has been isolated. The possibility of co-transfer of CMS and a selectable chloroplast trait, streptomycin resistance, was investigated by marker rescue from irradiated protoplasts, a method described from this laboratory (Menczel et al. 1982). As the source of the CMS factor, the Nicotiana tabacum St-R701 line was chosen. This line is male sterile due to replacement of its original cytoplasm by that of N. megalosiphon, and carries a cytoplasmic streptomycin resistance mutation (Umiel 1979). Following fusion of irradiated (lethal dose) St-R701 protoplasts with untreated N. plumbaginifolia protoplasts, streptomycin-resistant clones were selected. Regenerated plants representing 27 clones were studied in detail. These plants have a N. plumbaginifolia nuclear genome and the resistance factor from the St-R701 parent. In each clone transfer of streptomycin resistance resulted in the transfer of St-R701 chloroplasts suggesting the involvement of chloroplast DNA in determining streptomycin resistance. All regenerated streptomycin resistant N. plumbaginifolia plants were male sterile. In plants from three clones, in addition to male sterile flowers, a few male fertile flowers were obtained.